Consider changing docker-compose to only run Solr and Postgres, not the Rails app

[marlo-longley]

See for example our setup in Stanford-Arclight: https://github.com/sul-dlss/stanford-arclight/blob/fa8810e684150f82e676fd3279c1ab159900b6cc/docker-compose.yml -- this was a very handy way to work, it was basically a swap out for solr_wrapper with Docker.

In this approach, Rails app still runs directly on the local machine, so you do not need to prefix commands with docker compose exec app. You are also able to run tests successfully. Since we aren't deploying this app via Docker, removing it from the docker setup is a more straightforward dev setup in my opinion.

Writing up this ticket after a discussion with Nick and some minor work in #876